1. Field of the Invention
The present invention relates to an automatic transmission for an automotive vehicle, in particular, to an engaging force control device of lock-up clutch provided in a fluid coupling such as a torque converter.
2. Related Art
Generally, an automatic transmission provided in an automobile is constructed by a combination of a toque converter as a fluid coupling and a shift gear mechanism so as to switch a power transmitting path of the shift gear mechanism by means of a selective operation of a plurality of frictional elements such as clutches and brakes to thereby automatically establish a predetermined shift stage. It has been known that the torque converter is provided with a lock-up clutch which connects directly an input element from an engine with an output element to the shift gear mechanism.
Namely, the torque converter is provided with a pump (the input element) connected with an engine output shaft and integrally rotating with the shaft, a turbine (the output element) disposed facing to the pump and a stator disposed between the pump and the turbine to amplify a torque so as to transmit a rotation of the pump to the turbine through a operation fluid or hydraulic fluid and output the rotation transmitted from the pump to the shift gear mechanism through the turbine and the turbine shaft. In addition, the torque converter is provided with the lock-up clutch as a frictional member between the pump unit and the turbine unit. When the lock-up clutch is engaged, the input member of the torque converter or the output member of the engine is directly connected with the output member of the torque converter, namely the turbine or turbine shaft so as to rotate them integrally.
In this case, a property map is provided for determining the engagement or disengagement (release) of the lock-up clutch based on predetermined parameters of vehicle running conditions such as a throttle opening and a vehicle speed. The engaging condition of the lock-up clutch optimized to a current running condition of the vehicle is determined based on the property map of the detection of the running condition.
For example, when the vehicle is in a high engine and low vehicle speed condition, the lock-up clutch is disengaged or released in order to amplify the engine torque and absorb a shock during the shift operation to establish so called a converter condition.
While, when the vehicle is in a low engine load and high vehicle speed condition in which the above need is not so highly required, the lock-up clutch is fully engaged to establish so called a lock-up condition so as to enhance a power transmitting efficiency to thereby improve an engine fuel consumption performance.
Further, it has been known that the property map is provided with a slip zone in which the lock-up clutch is engaged with a certain amount of slip between the input and output elements the torque converter. In the slip condition, a slip control in which the slip amount is controlled to a predetermined target slip amount is carried out to absorb a vibration and the like while maintaining a desirable fuel consumption efficiency.
Meanwhile, when the vehicle runs into an uphill road, a running resistance against the vehicle running is increased due to a gradient of the uphill road to cause a speed reduction of the vehicle running. Under this condition, a driver tends to increase an acceleration pedal stroke, or increase a throttle opening to maintain the vehicle speed regardless of the gradient of the road. In this case, when the vehicle operating condition is shifted from the lock-up condition or slip condition to the converter condition in response to the increase of the throttle opening in connection with the stroke operation of the acceleration pedal, the lock-up clutch is disengaged so that an engine speed or engine driving force may be abruptly increased to cause an uncomfortable feeling for the driver.
In view of this problem, it has been proposed that the target slip amount is increased in response to the increase the road surface gradient of the vehicle running. According to the proposal, the target value of the slip amount is increased in the uphill road. Therefore, even though the vehicle operating condition is shifted from the slip condition to the converter condition due to the increase of the throttle opening, an abrupt fluctuation of the engine speed is suppressed because the difference between the actual slip amount in the slip control and the slip amount on the release of the lock-up clutch is reduced. On the other hand, the target slip amount is immediately increased on running into the uphill road. The actual slip amount is increased to achieve the target value without any stroke operation of the acceleration pedal and to increase the engine speed. As a result, the driver may feel uncomfortable as well under this condition.
In another aspect, when the acceleration pedal is stroked to compensate an insufficient driving power due to the increase of the running resistance resulting from the gradient of the road surface in the case where the vehicle runs into an uphill road in the course of the acceleration, the vehicle operating zone is shifted from the lock-up zone or slip zone to the converter zone so that the lock-up clutch is released to increase the driving power. When the acceleration pedal is stroked back to suppress an undue increase of the driving power, the running condition is shifted back to the lock-up zone or slip zone to engage the lock-up clutch to cause the insufficient driving power. Thus, a hunting phenomenon is caused with regard to the driving power of the engine.
In order to deal with the above problem, a Japanese Patent Unexamined publication No. 9-60718 which has been issued as a U.S. Pat. No. 5,807,204 discloses that once the throttle opening is shifted from a predetermined slip zone to a converter zone during the uphill running, the shift from the converter to the slip condition is prohibited. According to this idea, once the lock-up clutch is released during the uphill running, the lock-up clutch is maintained at the released condition. That is, the repetition of the engagement and the disengagement of the lock-up clutch is prevented to avoid the above problem.
However, the above technique described in the U.S. patent would produce the following problem due to the fact that the slip control is not stopped until the acceleration stroke or the throttle opening is increased beyond a predetermined value.
Namely, When the acceleration pedal is stroked to accelerate the vehicle for some reason when the vehicle runs on an uphill road in the slip control condition of the lock-up clutch, the slip condition is not stopped until the throttle opening is increased beyond a predetermined value and the lock-up condition is maintained. As a result, the torque converter does not exert the torque amplification function so that the driver may feel uncomfortable due to the shortage of the driving power.
Similar problem would occur when the acceleration pedal is stroked for acceleration during the lock-up control condition.